1. Field of the Invention
The present invention relates to an optical wavelength converter for shortening the wavelength of a laser beam, and more particularly to an optical wavelength converter element designed to prevent output variations in a wave whose wavelength has been converted.
2. Description of the Prior Art
As disclosed in, for example, U.S. Pat. No. 4,656,635, a solid-state laser is publicly known in which a solid-state laser crystal, doped with rare earth elements such as neodymium, is pumped with a semiconductor laser or the like. In this type of solid-state laser, in order to obtain a laser beam having a much shorter wavelength, it is widely practiced that the wavelength of a solid-state laser beam is converted into a second harmonic wave by disposing a crystal of a non-linear optical material in a resonator of the laser.
In the wavelength converter that converts the wavelength of the fundamental wave in the manner as mentioned above, a wave whose wavelength has been converted, such as a second harmonic wave, is emitted from both sides of the non-linear optical material, i.e., the side where the fundamental wave enters and the opposite side thereto. For this reason, the light transmission end faces of an element, arranged on the fundamental wave light source side with respect to the non-linear optical material such as the solid-state laser, are conventionally covered with coating that reflects a wavelength-converted wave, thereby leading to an improved wavelength conversion efficiency. In such a construction, the wavelength-converted wave that is emitted from the fundamental wave entrance side of the non-linear optical material is reflected by the coating and turns back to the non-linear optical material. This returning wave is then combined with another wavelength-converted wave that is emitted in the direction opposite to the fundamental wave entrance side, as a result of which a wavelength-converted wave having a high intensity is output.
However, in the construction n which a wavelength conversion efficiency is improved in the manner as mentioned above, if the wavelength of the fundamental wave varies for any reason, a phase difference between the two wavelength-converted waves, which are emitted from the non-linear optical material in directions opposite to each other, also changes. Interference between these two wavelength-converted waves resulting from this change in phase difference might lead to significant variations in an output of a wavelength-converted wave produced after the waves have been combined together. In some case, this interference leads to a substantially zero output of the wavelength-converted wave. In such a case, it is difficult to completely correct output variations of the wavelength-converted wave even when so-called APC (automatic power control) is effected.
When a crystal having a birefringence is used as a solid-state laser crystal, as has been conventionally practiced, a wavelength-converted wave is returned to the non-linear optical material by providing the end faces of the crystal with a high reflection coating with respect to the wavelength-converted wave. To avoid the separation of the wavelength-converted wave into an ordinary ray and an extraordinary ray in the non-linear optical material, it is necessary for the end surface of the crystal close to the non-linear optical material to be covered with a high reflection coating. In such a construction, variations in an output of the wavelength-converted wave are extremely large when the wavelength of the fundamental wave varies.